Embodiments of the invention relate generally to electronic power conversion and, more particularly, to a three-phase inverter based power system and method of controlling thereof that minimizes losses in the inverter during operation and maximizes overall system efficiency.
Photovoltaic (PV) power systems are power systems that employ a plurality of solar modules to convert sunlight into electricity. PV systems include multiple components, including photovoltaic modules, mechanical and electrical connections and mountings, and means of regulating or modifying the electrical output. One common arrangement in PV systems is for several PV modules to be connected in series to form a PV string, with multiple PV strings in a PV system then being combined in parallel to aggregate the current in a PV array. Photovoltaic (PV) cells generate direct current (DC) power, with the level of DC current being dependent on solar irradiation and the level of DC voltage dependent on temperature. For a typical one-stage solar inverter system, the DC output of the PV panel is fed to the DC bus of a power inverter through necessary isolation devices such as DC breakers, DC fuses, etc., with a DC pre-charge circuit often being employed to charge a DC bus capacitor during start-up. The AC output of the power inverter is then normally fed through line reactors and RC filters and is then connected to the grid through a main isolation transformer.
With respect to the power inverter, the inverter is often constructed as a two-level, three-phase inverter that includes six silicon carbide (SiC) MOSFETs each of which serves as a switch in the inverter. The SiC MOSFETs of the power inverter are typically controlled through pulse width modulation (PWM) to regulate the average inverter output voltage. The PWM is often generated by comparing a reference wave to a high-frequency carrier wave, with the carrier being a triangle waveform at a few kHz for solar power inverters. This switching frequency for the SiC MOSFETs can be higher than the switching frequency for traditional Si devices.
It is recognized that during operation of the PV power system, losses occur that are associated with the power inverter and other system components (e.g., reactor losses and filter losses), and that such losses can negatively impact the efficiency and performance of the PV power system. With respect to the power inverter losses, prior art systems have been designed to minimize such losses by utilizing a discontinuous pulse width modulation (DPWM) scheme for controlling switching of the SiC MOSFETs—with the DPWM providing for a non-switching period in the fundamental cycle and avoidance of switching at high current at differing power factors (if the phase angle of the DPWM reference waveform is controlled) in order to reduce switching losses in the power inverter.
However, while previously implemented techniques may have adequately addressed power inverter losses in PV power systems by utilizing a DPWM scheme where the phase angle of the DPWM reference waveform is controlled to reduce switching losses, such optimization of the power inverter operation may not result in minimizing overall losses in the PV power system. That is, while operation of the power inverter via DPWM with different phase angles will result in reducing switching losses in the power inverter, the use of different phase angles in the reference waveform results in different output current harmonic components being generated, and these current harmonic components can increase the level of line reactor losses and filter losses in the PV power system. Accordingly, the total PV power system losses may not be optimized if only the power inverter losses are optimized based on a determined operating power factor of the system.
It would therefore be desirable to provide a PV power system and method of operation thereof that utilizes DPWM to reduce power inverter losses in the system, but with the DPWM being applied and optimized to maximize the overall PV power system efficiency for full KVA range including low power factor operating conditions. It would further be desirable that, in maximizing the system efficiency, the SiC MOSFET switch stress would be effectively reduced and the reliability thereof would be increased.